Technical Field
This disclosure generally relates to methods and systems of driving light emitting diodes (“LEDs”). More particularly, the present disclosure relates to LED driver circuits that maintain an output voltage across an output capacitance element for an LED driver.
Description of Related Art
An LED is a P-N junction diode that emits light when a suitable voltage is applied to its leads. To that end, various circuits are used to power an LED. Such circuits not only provide sufficient current to light the LED at the desired brightness and color temperature, but also limit the current to prevent damaging the LED. FIG. 1A illustrates an example of a prior art LED driver system 100 that regulates output current 101 to LEDs 115 at a level indicated by a control signal at a control signal input 103 when a pulse width modulation (“PWM”) signal at the PWM node 105 is ON (i.e., HI). When the PWM signal is OFF (i.e., LO), the output current 101 is zero and the LED load 115 does not emit light. Accordingly, the average value of the output current 101 is controlled by the relative ON and OFF durations of the PWM signal. Put differently, the intensity of the light emitted by the LEDs 115 can be increased with a higher duty cycle and dimmed by lowering the duty cycle of the PWM signal at node 105.
As illustrated in FIG. 1A, an LED driver system 100 may include an LED driver 119, a voltage divider network that may include resistors 123 and 125 in series, an output capacitance element 117, a current sensor 121, and an electronic switch 111.
When the PWM signal 105 is OFF, the LED load 115 may be disconnected by the electronic switch 111 and the voltage that was across the output capacitance element 117 before the PWM signal 105 was turned OFF may be maintained by the output capacitance element 117.
The features of the LED driver system 100 may be better understood in view of FIG. 1B, which illustrates example waveforms of the LED driver system 100. When the PWM is ON, the LED load 115 is ON (e.g., emits light) and the voltage level at the output VOUT is determined by the sum of forward voltages of the LEDs 115 at the current level set by the control signal input 103 and regulated by the iLED feedback path via the current sensor 121. Consequently, the voltage level at the feedback node FB is determined by the resistors 123 and 125 for the given VOUT voltage. When the PWM is turned OFF, the LED load 115 is OFF (e.g., stops emitting light) and the voltages at the output VOUT as well as the feedback node FB are subject to leakage. When the PWM is turned back ON, the LED load 115 emits light, which may not be at the desired color temperature and/or intensity until the voltages at the output VOUT and feedback node FB are ramped back up to the appropriate level by the LED driver 119.
Ideally, the capacitance element 117 should hold the output voltage VOUT constant during the PWM OFF time. However, under real world conditions, the output capacitance element 117 decays (e.g., loses charge) during OFF periods of the PWM signal due to internal leakage and/or leakage of any circuits connected to the output capacitance element 117, such as the first electronic switch 111, feedback resistance elements (e.g., resistors) R1 (123) and R2 (125). The voltage drop becomes more significant as the PWM OFF duration increases. After a long PWM OFF time (e.g., more than 1 second), the output voltage VOUT across the output capacitance element 117 may be lower than its value of the PWM ON period.
Accordingly, when the PWM signal 105 is turned back ON after a long OFF period, the LED driver 119 may be subject to a recovery time until the output capacitance element 117 has returned to its original output voltage. Such a delay can be problematic in applications that desire the color temperature and/or the intensity of the LED load 115 to be at a predetermined level immediately after the LED load 115 is turned ON. Traditional approaches of having longer PWM ON time to include the recovery delay in addition to the desired LED load ON time, not only increases power consumption but may not be effective because the recovery delay may vary with the size of the output capacitance element 117, process, temperature, desired LED light intensity, and the PWM OFF durations.